The present invention relates generally to a method of hardening corrugating rolls and more particularly to a method of heat treating the flutes of corrugating rolls using a laser.
Corrugating rolls are typically used in machinery for manufacturing corrugated paperboard. The corrugating rolls are typically formed from an alloy steel, such as AISI 4340 steel, with a diameter of 12 inches and a length of 87 inches, and include longitudinally extending flutes which are used for forming the corrugations in paper. The number and size of the flutes may vary, however, corrugating rolls are usually provided with between 33 and 39 flutes having heights ranging from 0.187 to 0.142 inch.
The corrugating rolls typically have a hardness of approximately 40 R.sub.c, and the flute tips are subject to wear as a result of sliding contact with the paper being processed by the machine and must eventually be machined back to their original dimensions or the corrugating roll must be replaced with a new one. In order to alleviate this problem, the corrugating rolls are provided with a hardened outer surface.
Typical methods of hardening the surface of corrugating rolls include plating the outer portion of the roll with a hard outer covering such as chrome, heat treating the entire roll, or heat treating only the tips of the flutes where the major portion of the wear takes place, is described in U.S. Pat. No. 4,154,565, issued to Hyde et al and assigned to the assignee of the present application.
U.S. Pat. No. 4,154,565 discloses heat treating the flutes of a corrugating roll by means of either a laser or an electron beam. In the process described by the patent for heat treating the roll by using a laser, an average hardening depth of 0.031 inch was obtained. The depth to which a flute can be hardened in the process described by this patent is limited by the temperature to which the flute can be heated by the laser beam without melting the surface material of the flute which is related to the distribution of the power density through the cross section of the beam.
A laser emits a beam having a circular cross section with a power density which varies according to a Gaussian distribution such that the power density of the beam is greatest at the center of the beam and tapers off toward the radial edges. Thus, when a Gaussian laser beam is focussed upon the flute of a roll, the extent to which the flute may be heat treated by the beam is limited by the temperature which is reached by the portion of the flute impinged upon by the center of the beam, since this portion of the flute receives the greatest amount of energy in a given time and will be subject to reaching a melting temperature before the surrounding portions of the fute.
Therefore, there is a need for a method of heat treating the flutes of a corrugating roll to the maximum possible depth using a laser beam without causing surface meting or distortion of the flutes.